Semiconductor devices such as ICs and LSIs are typically fabricated by the following steps:
(a) prepare a lead material from a strip having a thickness of 0.1 to 0.3 mm;
(b) stamp out a lead frame conforming to the shape of the semiconductor device to be fabricated;
(c) apply high-purity Si or Ge semiconductor elements to selected areas of the lead frame by thermocompression with an electrically conductive resin such as Ag paste or by diffusion through a plating of Au, Ag, Ni or their alloys formed on one surface of the lead material and which plating is heated to 350.degree. C. or higher, or by soldering with a binary eutectic alloy such as Au-Si or Au-Ge;
(d) connect the semiconductor elements to the lead frame by gold wires (this is a bonding step);
(e) enclose with a protective plastic or ceramic package the semiconductor elements, gold wires and parts of the lead frame to which the semiconductor elements have been bonded;
(f) remove unnecessary parts from the lead frame to form discrete leads; and
(g) apply a soldering material to the legs of the leads to enable connection of the semiconductor device to the substrate.
The material of which the leads on semiconductor devices are made has to meet various requirements: (1) good stampability; (2) heat resistance high enough to avoid deformation and softening due to heat resulting from bonding leads to semiconductor devices; (3) good heat dissipation and electrical conductivity; and (4) strength high enough to avoid breakage due to bending and other mechanical shocks that may be encountered during shipment of the semiconductor devices or their assembling into electrical machines. In terms of characteristic values, the material is required to have a tensile strength of at least 28 kg/mm.sup.2, a Vickers hardness of at least 80, an elongation of at least 2%, an electrical conductivity (which is indicative of thermal dissipation or conductivity) of at least 2% IACS, and a softening point (a measure of thermal resistance) of at least 350.degree. C. Conventional materials that satisfy these requirements are Cu alloys such as Fe-doped copper, phosphor bronze and Ag-doped copper.
These materials have high electrical conductivity and hence good heat dissipation, but their resistance to oxidation is so low that leads made of them form an oxide film when they are fusion-bonded to semiconductor devices or when the latter are packed with plastics. This oxide film must be removed before the leads are coated with a soldering material or soldered to the substrate of each semiconductor device. But in order to remove the oxide film, ordinary fluxes which have corrosive effects on the semiconductor device should be replaced by non-corrosive fluxes. However, non-corrosive fluxes are very ineffective and are not capable of completely removing the oxide film. Therefore, almost all semiconductor manufacturers depend on chemical treatments such as cleaning with acids to remove the oxide film. But this method has one fatal defect: it is very difficult to treat only the leg portion of leads with acid solutions and this causes problems in regard to quality control, yield and cost.